1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus such as a copier, a facsimile machine, or a printer.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction devices having two or more of copying, printing, scanning, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of a latent image bearing member (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
To form images, the above-described image forming apparatuses may employ a negative-positive process. In the negative-positive process, the surface of the photoconductor is evenly charged by the charger, and then a potential at a portion on the evenly-charged surface of the photoconductor where an image is to be formed is reduced by electrostatic latent image forming means to form an electrostatic latent image on the surface of the photoconductor. Thereafter, toner charged to a polarity identical to a polarity of the charged surface of the photoconductor is applied to the electrostatic latent image using developing means to form a toner image. Alternatively, the image forming apparatuses may employ a positive-positive process to form images. In the positive-positive process, the surface of the photoconductor is evenly charged by the charger, and then a potential at a portion on the evenly-charged surface of the photoconductor where an image is not to be formed is reduced by the electrostatic latent image forming means to form an electrostatic latent image on the surface of the photoconductor. Thereafter, toner charged to a polarity opposite the polarity of the charged surface of the photoconductor is applied to the electrostatic latent image using the developing means to form a toner image.
When a recording medium electrostatically attracted to a recording medium conveyance member passes a transfer area facing the surface of the photoconductor, a transfer bias having a polarity that is the opposite of the polarity of the toner is supplied from transfer bias application means so that the toner image formed on the surface of the photoconductor by the negative-positive process or the positive-positive process is transferred onto the recording medium. Thereafter, the toner image is fixed to the recording medium by fixing means, and the recording medium having the fixed toner image thereon is discharged from the image forming apparatuses.
One longstanding problem of the above-described image forming apparatuses is the occurrence of paper jams when the recording medium passing through the transfer area is not removed from the surface of the photoconductor. Ordinarily, the recording medium passing through the transfer area is charged with the transfer bias, or dielectrically polarized, so that a front side of the recording medium having the toner image thereon has a polarity opposite the polarity of the surface of the photoconductor after passing through the transfer area. Accordingly, the recording medium passing through the transfer area is electrostatically attracted to the surface of the photoconductor passing through the transfer area at the same time as the recording medium, and is conveyed in a direction corresponding to a curvature of the photoconductor. At this time, because a resilience of the recording medium overcomes a force that electrostatically attracts the recording medium to the surface of the photoconductor, the recording medium is removed from the surface of the photoconductor and is appropriately conveyed to the fixing means and so forth.
However, when the force of electrostatic attraction is greater than the resilience of the recording medium, the recording medium is not removed from the surface of the photoconductor, causing a paper jam.
An example of several solutions for the above-described problem is provision of a separation pick on a downstream side from the transfer area relative to a direction of rotation of the surface of the photoconductor. A tip portion of the separation pick contacts the surface of the photoconductor, and accordingly, a leading edge of the recording medium electrostatically attracted to the surface of the photoconductor after passing through the transfer area is scratched by the tip portion of the separation pick. As a result, the recording medium is removed from the surface of the photoconductor and is conveyed to an appropriate conveyance path.
However, because the tip portion of the separation pick contacts the surface of the photoconductor, residual toner on the surface of the photoconductor gets attached to the tip portion of the separation pick. Consequently, when the recording medium is removed from the surface of the photoconductor using the separation pick, the residual toner attached to the tip portion of the separation pick gets further attached to the leading edge of the recording medium, causing blurring at the leading edge of the recording medium, or unnecessary lines on the image formed on the recording medium. Further, when being removed by the tip portion of the separation pick, a rear edge of the recording medium rapidly moves toward the recording medium conveyance member due to a loss of the force that electrostatically attracts the recording medium to the surface of the photoconductor. Consequently, the toner attached to the rear edge of the recording medium is scattered, blurring the image formed at the rear edge of the recording medium. In particular, image blur tends to occur at the rear edges of A3-size recording media.
Further, when the separation pick is deformed or abraded, it is difficult to remove the recording medium from the surface of the photoconductor using the separation pick. Consequently, a paper jam may occur at the separation pick, or the recording medium electrostatically attracted to the surface of the photoconductor may pass the separation pick and inadvertently conveyed to a cleaning device.
One approach to solve the above-described problems is to increase a surface resistivity of the recording medium conveyance member to 108Ω/□ or greater. Accordingly, a larger amount of charge can be retained on a surface layer of the recording medium conveyance member, so that a force that electrostatically attracts the recording medium to the recording medium conveyance member becomes greater than the force that electrostatically attracts the recording medium to the surface of the photoconductor. As a result, because the recording medium is electrostatically attracted and conveyed by the recording medium conveyance member, the separation pick is not necessary for removing the recording medium from the surface of the photoconductor, preventing the above-described problems caused by use of the separation pick.
However, upon close examination by the inventors of the present invention, it has been discovered that in the case of image forming apparatuses in which scumming caused by attachment of toner to a portion of the surface of the photoconductor where an image is not to be formed rarely occurs, an increase in the surface resistivity of the recording medium conveyance member by itself is not effective for removing the recording medium from the surface of the photoconductor. The toner attached to such portion of the surface of the photoconductor prevents the recording medium from being electrostatically attracted to the surface of the photoconductor in image forming apparatuses in which scumming often occurs on the surface of the photoconductor. As a result, the recording medium is attracted to the recording medium conveyance member by the charges on the surface layer of the recording medium conveyance member. By contrast, in image forming apparatuses in which scumming rarely occurs on the surface of the photoconductor, the force that electrostatically attracts the recording medium to the surface of the photoconductor tends to be too large. Consequently, the charges on the surface layer of the recording medium conveyance member cannot cause the recording medium to be attracted to the recording medium conveyance member, and the recording medium is not removed from the surface of the photoconductor.
Another approach to remove the recording medium from the surface of the photoconductor is to provide a pre-transfer irradiating device (pre-transfer neutralizing means) in the image forming apparatuses. The pre-transfer irradiating device reduces the electric potential at all portions on the surface of the photoconductor that are to face the recording medium at the transfer area, so that such portions on the surface of the photoconductor are neutralized after development is performed by the developing means and before the toner image is transferred onto the recording medium at the transfer area. Because the potential at such portions is neutralized by the pre-transfer irradiating device in advance before transfer, the force that electrostatically attracts the recording medium to the surface of the photoconductor after the recording medium passes through the transfer area may be reduced. As a result, the ability to remove the recording medium passing through the transfer area from the surface of the photoconductor (hereinafter simply referred to as removability of a recording medium) is enhanced, preventing the above-described problems.
However, because all portions on the surface of the photoconductor that are to face the recording medium at the transfer area are neutralized by the pre-transfer irradiating device, light-induced fatigue of the photoconductor is accelerated, thus shortening the service life of the photoconductor.
Further, in the negative-positive process, movement of the toner attached to the electrostatic latent image formed on the surface of the photoconductor in a direction along the surface of the photoconductor is restricted by a magnetic field generated between the electrostatic latent image and a portion other than the electrostatic latent image charged to a polarity identical to the polarity of the toner. However, because all portions on the surface of the photoconductor that are to face the recording medium at the transfer area are neutralized, the potential at portions other than the electrostatic latent image adjacent to the electrostatic latent image is evenly decreased. Consequently, the magnetic field between the electrostatic latent image and the portion other than the electrostatic latent image is decreased, and a repulsive force between toner having the same polarity is increased. As a result, the toner is scattered on the surface of the photoconductor before transfer, causing image deterioration including blur.
To solve the above-described problems, an image forming apparatus in which electrostatic attraction of only a portion on the surface of the photoconductor corresponding to the leading edge of the recording medium is decreased has been proposed to achieve enhanced removability of the recording medium from the surface of the photoconductor. Specifically, only the portion on the surface of the photoconductor corresponding to an area between the leading edge of the recording medium and 2 or 3 mm ahead of the leading edge (hereinafter referred to as a leading edge area) is neutralized. Accordingly, the potential at portions other than the electrostatic latent image on the surface of the photoconductor corresponding to the leading edge area of the recording medium is reduced by neutralization, decreasing the force that electrostatically attracts the leading edge area of the recording medium to the surface of the photoconductor. As a result, the leading edge of the recording medium is removed from the surface of the photoconductor, and a paper jam can be prevented even when a recording medium having a higher resilience is used. Further, because the portion to be neutralized by the pre-transfer irradiating device can be reduced as described above, light-induced fatigue of the photoconductor is suppressed, preventing acceleration of deterioration of the photoconductor.
It is to be noted that when the negative-positive process is employed in the above-described image forming apparatus, a potential at portions other than the electrostatic latent image on the surface of the photoconductor corresponding to portions other than the leading edge area of the recording medium, that is, almost all portions of the recording medium, is not reduced. As a result, the toner attached to the electrostatic latent image formed at portions on the surface of the photoconductor corresponding to portions other than the leading edge area of the recording medium is not scattered. In other words, toner scattering can be prevented at portions on the surface of the photoconductor corresponding to almost all portions of the recording medium, preventing image deterioration.
Further, in the above-described image forming apparatus, a transfer bias is decreased only when the portion on the surface of the photoconductor corresponding to the leading edge area of the recording medium is positioned at the transfer area compared to a transfer bias applied when portions on the surface of the photoconductor corresponding to portions other than the leading edge area of the recording medium are positioned at the transfer area. Accordingly, an amount of charge supplied to the leading edge area of the recording medium is reduced or eliminated, and the force that electrostatically attracts the leading edge of the recording medium to the surface of the photoconductor is further reduced. As a result, even a recording medium having a lower resilience can be reliably removed from the surface of the photoconductor.
However, it has been confirmed by the inventors of the present invention that the removability of the recording medium from the surface of the photoconductor cannot be reliably provided over time using the approaches described above. Upon close inspection, it has been found that the potential at the surface of the photoconductor cannot be sufficiently reduced by neutralization due to deterioration of the photoconductor over time. Specifically, when the potential at the surface of the photoconductor cannot be sufficiently reduced, the portion on the surface of the photoconductor corresponding to the leading edge area of the recording medium cannot be sufficiently neutralized by the pre-transfer irradiating device over time. In a widely-used image forming apparatus, when the potential at the surface of the photoconductor cannot be sufficiently reduced by neutralization, the potential at the surface of the photoconductor is increased by performing image adjustment such as process control to provide higher-quality images. Consequently, the potential at the portion on the surface of the photoconductor corresponding to the leading edge area of the recording medium cannot be sufficiently neutralized by the pre-transfer irradiating device. As a result, the leading edge of the recording medium tends not to be removed from the surface of the photoconductor.
One possible solution to the above-described problem is to increase an amount of light to be directed onto the surface of the photoconductor from the pre-transfer irradiating device (hereinafter referred to as an amount of radiation) so that the potential at the surface of the photoconductor can be sufficiently reduced even when the photoconductor deteriorates over time.
However, use of too great amount of radiation from an initial stage of use of the photoconductor accelerates deterioration of the photoconductor. Further, formation of images at the leading edge area of the recording medium has come to be demanded of image forming apparatuses, and when the amount of radiation is increased at the initial stage, deterioration of the photoconductor is accelerated. In image forming apparatuses employing the negative-positive process, the portion on the surface of the photoconductor corresponding to the leading edge area of the recording medium is over-neutralized. Consequently, toner scattering easily occurs when the image is formed at the leading edge area of the recording medium, possibly causing blurring of the image formed at the leading edge area of the recording medium.
In the above-described case in which the transfer bias is decreased when the portion on the surface of the photoconductor corresponding to the leading edge area of the recording medium is positioned at the transfer area (hereinafter referred to as a leading edge transfer bias) to provide reliable removability of the recording medium from the surface of the photoconductor, the potential at the surface of the photoconductor cannot be sufficiently reduced by neutralization due to deterioration of the photoconductor. Consequently, the potential at the surface of the photoconductor is increased by performing process control, preventing reliable removability of the recording medium from the surface of the photoconductor.
One possible way to provide reliable removability of the recording medium from the surface of the photoconductor even when the potential at the surface of the photoconductor is increased over time is to decrease the leading edge transfer bias. However, when the leading edge transfer bias is too low, the toner image formed at the leading edge area of the recording medium cannot be satisfactorily transferred onto the recording medium at the initial stage of use of the photoconductor.